1. Technical Field
The present disclosure relates to a power metal oxide semiconductor field effect transistor, in particular, to a double gate trench power transistor.
2. Description of Related Art
Power Metal Oxide Semiconductor Field Transistor (Power MOSFET) is widely used as a switch element of electrical devices such as a power supply, rectifier or low pressure motor controller, etc. Most of the existing power metal oxide semiconductor field transistors adopt vertical structure design to increase the component density. These power metal oxide semiconductor field transistors having vertical structure design also refer to double gate power metal oxide semiconductor field transistors, and the advantage thereof is that it would be able to control the voltage for operating the element under low power consumption.
The parameters that affect the element properties of the power metal oxide semiconductor field transistors include the source/drain on-resistance (Rdson), breakdown voltage and switching speed, etc. However, regarding the power metal oxide semiconductor field transistors, the source/drain on-resistance (Rdson) and the breakdown voltage are in positive correlation. In other words, while increasing the doping concentration of the drift region or reducing the thickness of the drift region for reducing the source/drain on-resistance (Rdson), the breakdown voltage would be reduced as well.
Therefore, in order to maintain higher breakdown voltage of the power metal oxide semiconductor field transistors under relatively low source-drain on-resistance, the trend is to form a shielding electrode structure in the gate trench that extends into the drift region.
In some of the structures of the power metal oxide semiconductor field transistor, two gate electrodes and a shielding electrode parallel are arranged in the same trench and electrical insulated by an oxide layer. Furthermore, the shielding electrode is positioned between the two gate electrodes and extends into the drift region through the surface of the epitaxial layer.
However, during the manufacture of the above power metal oxide semiconductor field transistor, the step of forming the gate oxide layer and the step of forming the insulating layer for isolating the gate electrodes and the shielding electrodes are performed in the same process, therefore, the thickness of the insulating layer between the gate electrodes and the shielding electrode is relatively small, thereby increasing the resistance between the gate and the source.
Furthermore, subjected to the limitation of the process conditions, the bottom oxide layer is hard to deposit and is relatively thin, especially the side close to the shielding electrode at the bottom of the gate electrode. Therefore, the point potential effect easily occurs, thereby reducing the withstand voltage of the gate and inducing the degradation of the gate under high temperature. Accordingly, the reliability of the power metal oxide semiconductor field transistor would decrease thereby affecting the lifetime of the power metal oxide semiconductor field transistor.